1. Technical Field of the Invention
This invention is related to current transformers, including error compensation for improving output accuracy of current transformers.
2. State of the Prior Art
Current transformers are electrical devices that can provide a small, measurable current or voltage output signal that is indicative of a larger current flowing in an electric line, so they are often used as a component in electrical metering, monitoring, recording, and control instruments where large, high power, transmission or load situations would make direct measurements of electric current impractical or unsafe. Current transformers also isolate the measuring instruments from high voltages in such high power conductors or circuits.
Of course, accuracy and reliability are always at least of some concern in measuring devices, depending the applications and uses of the measurements. For current transformers, especially those used in revenue metering instruments where customers or users may be charged based on the amount of electric power used, the accuracy of the current transformer output signals for measuring current flowing in the electric line, thus electric power delivered by the electric line or used by a load connected to the electric line, is very important. Customers do not want to be charged for electric power that they do not use, and electric utility providers want to be sure that they are charging for all the power that a customer uses.
However, current transformers have inherent physical characteristics that result in current measurement errors, including ratio errors and phase angle errors, both of which affect the accuracy of current measurements made with current transformers. Ideally, the output signal of a current transformer is a specific ratio to the input current of a primary winding, for example, a primary winding in the form of a high power electric line, where the ratio is equal to the ratio number of turns of the wire that forms the primary winding to the number of turns of the wire that forms the secondary winding of the current transformer. However, a number of physical characteristics of the current transformer, such as the magnetic core materials, core construction, electrical resistances and reactances, and other parameters result in the output signals being somewhat less than the ideal ratio relationship to the input current being measured. Such ratio error results in the output signals of current transformers being somewhat less than accurate indicators or measurements of the input current. Ideal output signals would also be exactly in phase with the input current. However, some of the same physical characteristics that cause ratio errors in current transformers also cause the output signals to be somewhat out of phase with the input current being measured. Such phase angle errors do not cause significant accuracy problems for measurement of current, but, if the output measurements are used for measuring electric power, such phase angle errors can be very significant and can cause significant accuracy issues for electric power measurements and metering. Since public utilities charge customers for electric power used, measuring and metering electric power with current transformers that have even small phase angle errors may not have sufficient accuracy to meet such electric power and metering needs.
Persons skilled in the art know that increasing inductance and reducing resistance of current transformers can improve accuracy and that more turns of the wire in the secondary winding will provide more inductance. However, increasing the number of turns also requires more wire, thus also increases resistance, and more turns with more wire causes the physical size to be larger. If keeping the physical size small is a design criterion, more turns could be accommodated with thinner wire to keep physical size small, but thinner wire would also result in more resistance. Therefore, it is difficult to provide more inductance and at the same time reduce resistance.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to persons skilled in the art upon a reading of this material.